1. Field of the Invention
This invention relates to a process for improving the properties of metal oxide superconductors said process comprising depositing an alkali metal onto the surface.
2. References
Bednorz and Muller, Z. Phys. B64, 189 (1986), disclose a superconducting phase in the La-Ba-Cu-O system with a superconducting transition temperature of about 35 K. This disclosure was subsequently confirmed by a number of investigators [see, for example, Rao and Ganguly, Current Science, 56, 47 (1987), Chu et al., Science 235, 567 (1987), Chu et al., Phys. Rev. Lett. 58, 405 (1987), Cava et al., Phys. Rev. Lett. 58, 408 (1987), Bednorz et al., Europhys. Lett. 3, 379 (1987)]. The superconducting phase has been identified as the composition La.sub.1-x (Ba,Sr,Ca).sub.x CuO.sub.4-y with the tetragonal K.sub.2 NiF.sub.4 -type structure and with x typically about 0.15 and y indicating oxygen vacancies.
Wu et al., Phys. Rev. Lett. 58, 908 (1987), disclose a superconducting phase in the Y-Ba-Cu-O system with a superconducting transition temperature of about 90 K. Cava et al., Phys. Rev. Lett. 58, 1676 (1987), have identified this superconducting Y-Ba-Cu-O phase to be orthorhombic, distorted, oxygen-deficient perovskite YBa.sub.2 Cu.sub.3 O.sub.9-.delta. where .delta. is about 2.1 and present the powder x-ray diffraction pattern and lattice parameters.
C. Michel et al., Z. Phys. B-Condensed Matter 68, 421 (1987), disclose a novel family of superconducting oxides in the Bi-Sr-Cu-O system with composition close to Bi.sub.2 Sr.sub.2 Cu.sub.2 O.sub.7+.delta.. A pure phase was isolated for the composition Bi.sub.2 Sr.sub.2 Cu.sub.2 O.sub.7+.delta.. The X-ray diffraction pattern for this material exhibits some similarity with that of perovskite and the electron diffraction pattern shows the perovskite subcell with the orthorhombic cell parameters of a=5.32 A (0.532 nm), b=26.6 A (2.66 nm) and c=48.8 A (4.88 nm). The material made from ultrapure oxides has a superconducting transition with a midpoint of 22 K as determined from resistivity measurements and zero resistance below 14 K. The material made from commercial grade oxides has a superconducting transition with a midpoint of 7 K.
H. Maeda et al., Jpn. J. Appl. Phys. 27, L209 (1988), disclose a superconducting oxide in the Bi-Sr-Ca-Cu-O system with the composition near BiSrCaCu.sub.2 Ox and a superconducting transition temperature of about 105 K.
The commonly assigned application, "Superconducting Metal Oxide Compositions and Process For Making Them", Ser. No. 153,107, filed Feb. 8, 1988, a continuation-in-part of Ser. No. 152,186, filed Feb. 4, 1988, now abandoned, disclose superconducting compositions having the nominal formula Bi.sub.a Sr.sub.b Ca.sub.c Cu.sub.3 O.sub.x wherein a is from about 1 to about 3, b is from about 3/8 to about 4, c is from about 3/16 to about 2 and x=(1.5 a+b+c+y) where y is from about 2 to about 5, with the proviso that b+c is from about 3/2 to about 5, said compositions having superconducting transition temperatures of about 70 K or higher. It also discloses the superconducting metal oxide phase having the formula Bi.sub.2 Sr.sub.3-z Ca.sub.z Cu.sub.2 O.sub.8+w wherein z is from about 0.1 to about 0.9, preferably 0.4 to 0.8 and w is greater than zero but less than about 1. M. A. Subramanian et al., Science 239, 1015 (1988) also disclose the Bi.sub.2 Sr.sub.3-z Ca.sub.z Cu.sub.2 O.sub.8+w superconductor.
Z. Z. Sheng et al., Nature 332, 55 (1988) disclose superconductivity in the Tl-Ba-Cu-O system in samples which have nominal compositions Tl.sub.2 Ba.sub.2 Cu.sub.3 O.sub.8+x and TlBaCu.sub.3 O.sub.5.5+x. Both samples are reported to have onset temperatures above 90 K and zero resistance at 81 K. The samples were prepared by mixing and grinding appropriate amounts of BaCO.sub.3 and CuO with an agate mortar and pestle. This mixture was heated in air at 925.degree. C. for more than 24 hours with several intermediate grindings to obtain a uniform black oxide Ba-Cu oxide powder which was mixed with an appropriate amount of Tl.sub.2 O.sub.3, completely ground and pressed into a pellet with a diameter of 7 mm and a thickness of 1-2 mm. The pellet was then put into a tube furnace which had been heated to 880.degree.-910.degree. C. and was heated for 2-5 minutes in flowing oxygen. As soon as it had slightly melted, the sample was taken from the furnace and quenched in air to room temperature. It was noted by visual inspection that Tl.sub.2 O.sub.3 had partially volatilized as black smoke, part had become a light yellow liquid, and part had reacted with Ba-Cu oxide forming a black, partially melted, porous material.
Z. Z. Sheng et al., Nature 332, 138 (1988) disclose superconductivity in the Tl-Ca-Ba-Cu-O system in samples which have nominal compositions Tl.sub.2 Ca.sub.2 BaCu.sub.3 O.sub.9+x with onset of superconductivity at 120 K.
R. M. Hazen et al., Phys. Rev. Lett. 60, 1657 (1988), disclose two superconducting phases in the Tl-Ba-Ca-Cu-O system, Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 and Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub.8, both with onset of superconductivity near 120 K. C. C. Torardi et al., Science 240, 631 (1988) disclose the preparation of Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 with an onset of superconductivity of 125 K.
S. S. P. Parkin et al., Phys. Rev. Lett. 61, 750 (1988), disclose the structure TlBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9.+-.y with transition temperatures up to 110 K.
M. Hervieu et al., J. Solid State Chem. 5, 212 (1988), disclose the oxide TlBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8-y.
C. C. Torardi et al., Phys. Rev. B 38, 225 (1988), disclose the oxide Tl.sub.2 Ba.sub.2 CuO.sub.6 with an onset of superconductivity at about 90 K.
The commonly assigned application, "Superconducting Metal Oxide Compositions and Processes For Manufacture and Use", Ser. No. 236,088, filed Aug. 24, 1988, a continuation-in-part of Ser. No. 230,631, filed Aug. 10, 1988, now abandoned, disclose superconducting compositions having the nominal formula Tl.sub.e Pb.sub.a Ca.sub.b Sr.sub.c Cu.sub.d O.sub.x wherein a is from about 1/10 to about 3/2, b is from about 1 to about 4, c is from about 1 to about 3, d is from about 1 to about 5, e is from about 3/10 to about 1 and x=(a+b+c+d+ e+y) where y is from about 1/2 to about 3. These compositions have an onset of superconductivity of at least 70 K.
J. M. Liang et al., Appl. Phys. Lett. 53, 15 (1988) disclose a composition TlBa.sub.2 Ca.sub.3 Cu.sub.4 O.sub.x with an onset of superconductivity at 155 K and a zero resistance at 123 K. CaCO.sub.3, BaCO.sub.3 and CuO powders were ground together and calcined for 15 hours with intermediate grindings. The Ba-Ca-Cu-O powders were mixed with Tl.sub.2 O.sub.3 to yield a mixture with nominal composition TlBaCa.sub.3 Cu.sub.3 O.sub.x. This mixture was ground, pressed and sintered for 15 minutes in flowing O.sub.2. Composition ratios of the Tl:Ca:Ba:Cu in the superconductor vary from 1:2:2:3 to 1:2:3:4.